In the context of road reconstruction by milling, and the removal of mineral deposits with a surface miner (also by milling), the earth working tools that are used, and in particular the milling bits, are subject to a continuous wear process. Replacement is advisable once the tools reach a specific wear state, since otherwise the ongoing process loses efficiency. A distinction must be made here among a variety of wear states that result in replacement of a milling bit or bit holder:    1. Bit replacement because sufficient wear material (in particular carbide metal at the tip) is no longer present. The penetration resistance becomes too high and the efficiency thus decreases (too much frictional loss); the wear is predominantly rotationally symmetrical.    2. Bit holder replacement because the wear limit has been reached (wear occurs on the holder at the contact surface between bit and holder). This wear is usually symmetrical.    3. Rotationally asymmetrical wear on the bit tip and/or bit head due to insufficient rotational movement of the bit during the milling process. The consequences are a poor milling pattern as well as the risk of tool breakage, since the bracing effect of the bit head is lost.    4. The bit holder can furthermore be subject to additional rotationally asymmetrical wear.    5. Bit breakage.
In addition, worn and/or broken bits can result in secondary damage to the bit holders, and worn bit holders can cause secondary damage to the milling drum. Timely replacement of the bit and/or bit holder is therefore necessary and reduces costs. If the bits and/or bit holders are changed too soon, however, this procedure is likewise not optimum in terms of cost, since the bits and bit holders are consumable parts and therefore very cost-intensive. Wear potential that is still available is then not being correctly utilized. The wear state of bits and bit holders has hitherto been assessed by visual inspection by the machine driver. For this purpose the machine operator must shut off the machine (switch off engine and decouple drum from the drive train). He must then open the rear drum hatch so the milling drum can be visually inspected.
The milling drum is then rotated by means of a second drive system so that the entire milling drum can be inspected. The drum inspection task can also be handled by a second operator. The wear state of the bit holders is usually assessed by way of so-called “wear markings.” The wear state of the bits can be determined by way of the longitudinal wear and the rotational symmetry of the wear pattern.
Monitoring the wear state of bits and holders is very time-intensive, and is unproductive since nothing can be produced during that time. The overall process is disrupted and availability is thus additionally decreased. In addition, because of the highly subjective nature of the assessment, the risk exists that the wear potential of holders and bits will not be optimally utilized.
DE 102 03 732 A1 (U.S. Pat. No. 7,422,391) discloses an apparatus in which operation can be optimized by monitoring operating states of machine components that participate directly or indirectly in the milling process. Among other factors, the wear state of the bits is also assessed by evaluating a variety of machine parameters and variables. The problem that exists during operation of the milling machine is that the milling process and the substrate itself, whose properties fluctuate considerably, have a large influence on evaluation of the operating state of components.
AT 382 683 B discloses a mining machine in which the cutting drum is monitored in noncontact fashion, using photoelectric barriers that detect the presence of the bits. A quantitative wear evaluation is not possible with this method.
DE 10 2008 045 470 A1 (U.S. Pat. No. 8,386,196) discloses a method for quantitative wear identification. Here the position in space of at least point on the earth working tool is sensed. This measurement result is then compared with a reference value so that the wear on the tool can be sensed quantitatively.
As already mentioned above, the material properties of the substrate being processed change during the working process. In mining, for example, it can happen that while traveling over a deposit of material to be removed, the hardness of the raw material suddenly rises (“hard spot”). Increased wear then occurs on the tools.
In order to avoid damage to the cutting equipment due to unpredictable working conditions, for safety reasons the tools are on occasion switched out too soon.